The present invention relates to a new method for supporting metal and/or metal oxide on crystalline aluminosilicate zeolite. The catalysts produced by this method are also new and shall hereinafter be designated ZKU-5, ZKU-6, and ZKU-7.
The above catalysts have superior catalytic activity, selectivity, and catalytic life and are suitable for use in the manufacture of low molecular weight olefins from methanol.
Two such olefins in high demand are ethylene and propylene, which are used as the main starting materials in the petrochemical industry and which are preferred over the use of petroleum naphtha.
Heretofore, crystal aluminosilicate zeolite catalyst have been known. Examples of such may be found in Japanese patent application no. 136,715/80, now Japanese Patent laid open No. 63,135/82 wherein the catalysts are designated ZKU-2, ZKU-3, and ZKU-4. These catalysts are manufactured by adding a tetraloweralkyl ammonium cation and a crystal seed to synthesis reaction mixture. The period of time necessary to produce the crystal is remarkably shortened and the catalysts have superior catalytic activity, selectivity and a longer life.
The crystal seed is used to promote crystalization and catalytic activity and to improve mechanical strength and dimensional stability. Further, it provides a conditioning of the pore structure in the molded catalyst crystal and improves dispersion of the components in the catalytic composition.
Generally, there are three prior art methods available for depositing a metal and/or a metal oxide, viz., (a) ion exchange, (b) impregnation and (c) physical mixing. These are as follows:
(a) The ion exchange method PA0 The impregnation method PA0 (c) The physical mixing method
The ion exchange method is the most conventional of the three. However, it is limited in the type and quantity of metal which can be deposited. Metal oxides cannot be deposited directly using this method. Moreover, the ion exchange rate is difficult to control if more than two metallic ions are to be exchanged, due to their different ion exchange rates.
The ion exchange method also has an adverse effect on the cellular structure and the physical strength of the zeolite crystal.
The impregnation method is disadvantageous in that, by nonselectively covering the zeolite with the impregnating substance, the activity points and thus the catalystic activity of the zeolite is reduced. The activity is further affected by a reduction in the diffusion of the reaction components in the zeolite due to blockage of the pores.
This method involves compounding the metal catalysts while maintaining the catalytic activity of the zeolite itself. Exemplary of mixed catalysts made using this method are ZSM-5 zeolite catalysts recently manufactured by Mobil Company and the Fisher-Tropsch synthesis catalyst, both of which have been used in the manufacture of a synthesis gas product rich in the gasoline fraction using a one step process. In using the physical mixing method, however, there is some concern that the catalyst will exist in an uneven mixing state, since the method involves merely physically combining the components. The composite effect obtained from combining each component of the catalyst is also limited, since the catalytic activity points of the composite catalysts are too far removed from each other when considered from the standpoint of kinetics.
The characteristic features of the present invention are as follows:
(1) The "seed or nucleus crystallization method" of the invention is capable of uniting various sorts of metal oxide with the zeolite crystal without injuring the specific character of the activity or the porosity of the zeolite catalyst. This method overcomes the disadvantages of the impregnation and ion exchange methods.
(2) The method is also capable of uniting optimal catalytic components comprising one or not less than two metals and/or metal oxides at the optimal ratio, thus overcoming a disadvantage of the ion exchange method.
(3) The method is further capable of uniformly and highly dispersing one or not less than two metals and/or metal oxides on the individual zeolite crystal.
The method of the invention involves combining the metal and/or metal oxide directly with the individual zeolite crystal to produce a uniform and highly dispersed state, in contrast to similar catalysts manufactured according to the physical mixing method. As the crystal seed is combined with the metal and/or metal oxide and then intimately adhered to the zeolite crystal, each active metal and/or metal oxide component are in a position nearer to the zeolite crystal compared to a similar catalyst made using the physical mixing method. The composite effect of catalytic activity is thus fully displayed.
As mentioned above, the present invention overcomes the disadvantages of the physical mixing method.